[PATCH] Cygwin: add fd validation to mq_* functions

[Christian Franke]

Mark Geisert wrote:
> Validate the fd returned by cygheap_getfd operating on given mqd.
>
> Reported-by: Christian Franke <Christian.Franke@t-online.de>
> Addresses: https://cygwin.com/pipermail/cygwin/2025-January/257090.html
> Signed-off-by: Mark Geisert <mark@maxrnd.com>
> Fixes: 46f3b0ce85a9 (Cygwin: POSIX msg queues: move all mq_* functionality into fhandler_mqueue)
>
> ---
>   winsup/cygwin/posix_ipc.cc | 88 +++++++++++++++++++++++---------------
>   1 file changed, 53 insertions(+), 35 deletions(-)
>
> diff --git a/winsup/cygwin/posix_ipc.cc b/winsup/cygwin/posix_ipc.cc
> index 34fd2ba34..3ce1ecda6 100644
> --- a/winsup/cygwin/posix_ipc.cc
> +++ b/winsup/cygwin/posix_ipc.cc
> @@ -225,11 +225,14 @@ mq_getattr (mqd_t mqd, struct mq_attr *mqstat)
>     int ret = -1;
>   
>     cygheap_fdget fd ((int) mqd, true);
> -  fhandler_mqueue *fh = fd->is_mqueue ();
> -  if (!fh)
> -    set_errno (EBADF);
> -  else
> -    ret = fh->mq_getattr (mqstat);
> +  if (fd >= 0)
> +    {
> +      fhandler_mqueue *fh = fd->is_mqueue ();
> +      if (!fh)
> +        set_errno (EBADF);
> +      else
> +        ret = fh->mq_getattr (mqstat);
> +    }

Sorry, I forgot to mention that the testcase also "works" (segfaults) if 
a positive but nonexistent fd is used. I'm not sure whether the (fd >= 
0) check is sufficient.